feat(backend): add FastAPI backend — procurement, inventory domain, Casdoor auth, PM arrival endpoints

backend/app/utils/serialization.py

@@ -0,0 +1,16 @@
+"""JSON-safe serialization helpers (contract: ISO 8601 UTC with `Z`)."""
+from datetime import date, datetime, timezone
+
+
+def to_iso(value: datetime | None) -> str | None:
+    """Timestamptz -> '2026-08-02T10:00:00Z' (UTC). Naive values are treated as UTC."""
+    if value is None:
+        return None
+    if value.tzinfo is None:
+        value = value.replace(tzinfo=timezone.utc)
+    return value.astimezone(timezone.utc).isoformat().replace("+00:00", "Z")
+
+
+def to_date(value: date | None) -> str | None:
+    """Date -> 'YYYY-MM-DD'."""
+    return value.isoformat() if value is not None else None